Currently, most flat panel x-ray imagers are based on amorphous silicon (a-Si) thin-film transistor (TFT) sensor technology. While TFT flat panel imagers are capable of providing large field-of-view (FOV) images, they have limitations in resolution, low dose performance, and readout speed. Resolution is typically limited to approximately 70 μm for low speed mammography applications, and more typically 140 μm for radiographic imaging. In dynamic applications, in order to achieve reasonable low dose performance, larger effective pixel sizes of 180 μm to 400 μm are used, either in full resolution or binned modes of operation, with x-ray quantum-limited doses in the range of 3-10 nGy, dictated by the minimum electronic noise of approximately 1000e per pixel. Maximal readout speeds available are typically in the range of 15 frames per second (fps) for full resolution and 30 fps for binned resolution, limited by the speed of charge transfer in the pixel and the need to filter out electronic noise generated by the large parasitic data line capacitances in flat-panel imagers. Moreover, a-Si photodiodes are loaded with deep traps, which may lead to image ghosting artifacts.
Complementary metal-oxide-semiconductor (CMOS) image sensors have been developed that overcome limitations of resolution, low dose performance, and readout speed. Pixel amplifiers can boost the signals generated by x-ray photons, reducing the quantum limited doses by at least 10 times lower relative to a-Si TFT flat panels. While such amplifiers do take up significant space in a pixel, resolutions down to 50 μm can be readily achieved in this technology. Due to the much higher mobility of crystalline silicon (c-Si) and the lower pixel capacitances of photodiodes made in this technology, readout speeds are no longer limited by charge transfer in the pixel and higher bandwidth amplifiers can be used without compromising electronic noise. Readout speeds well above 100 fps are achievable with CMOS technology. Further, photodiodes made from c-Si do not typically contain significant levels of deep traps, and thus image ghosting originating from the photodiodes is absent in CMOS sensors.
CMOS image sensors are generally used in optical cameras and are not typically designed with high dynamic range and linearity requirements for X-ray imagers. X-ray sensors utilize full CMOS wafers (typically 8 or 12 inches in diameter) and are often tiled to achieve large-area formats. CMOS sensors can overcome many of the technical disadvantages of a-Si TFT flat panel detectors, but the cost of building a detector from multiple CMOS wafers is considered prohibitive for nearly all medical device applications.